It really doesn't matter whether their fight was a decade ago or a day ago because both Mihawk & Shanks were fighting other people after they stopped duelling. The Marines definitely know well enough to make a statement about it, just like how Greenbull said it for Kaido & Sengoku told it for WB.
And even then, on a meta lvl this is direct exposition drop from Oda on the topic about Mihawk & Shanks's PL.
it does matter or any significant battles would be hinted towards..! and during that decade who did Mihawk fight as a Warlord... he definitely didn't fight any Yonko's or even admirals as he was a Shichibukai.. are we talking about Yc's? are just going assume that he has confrontations with Yonko's and that weren't even recalled during Yonko saga? i mean to our knowledge he didn't even fight against Vista in that decade long time
Your claims goes against your stance itself...
and to say marines would go off of other fights that are likely not swords fights to gauge Mihawk's swordsmanship against Shanks which they also have Likely no account of.. is also a massive stretch and tbh just plain ridiculous.
Also with that route of thought shouldn't Shanks be over Mihawk coz
Shank clashed with 2 of the strongest Pirates in the NW.. made one retreat on top of that.
Its obvious that Marines are going off of retrospective duels and the fact that Shanks lost an arm given nothing of Shanks loosing against Mihawk was ever stated..
all in all,
- your argument proves the contrary to be the case
- Mihawk didn't fight anyone notable to really be mentioned in the story in that decade
- Shanks then isn't the Shanks now and Marines are basing things not on confrontations Mihawk and Shanks had but rather their duels a decade ago
- Marines wouldn't logically base their conclusion over the fight Mihawk and Shanks had in the decade... coz that would be even more inaccurate.. rather they would chose to go off of duels that were more accurate and its convenient to assume Shanks is less skilled with one arm and the fact that Mihawk is labeled the WSS for a reason.
- basing the conclusion of a hypothetical battle on dodgy and outdated accounts of duels isn't the least accurate